1. Field of the Invention
This invention relates to a magnetic tape drive, and in particular to a tape drive utilizing multiple capstans.
2. Description Relative to the Prior Art
A critical component in a magnetic tape recorder/reproducer system is the tape transport mechanism. The reproduced quality and fidelity of a previously recorded signal is highly dependent upon the uniformity of tape movement relative to the recorder heads both during record and playback, and this uniformity is primarily determined by the tape drive characteristics. The use of a metering capstan to provide uniform tape speed during recording and playback has been long known in the art, and many capstan configurations have been implemented in the search for the ideal uniform speed tape drive. This ideal capstan driven tape transport has not been attained; one reason is the perturbations in the tension experienced by the tape as it is transported from a supply reel past the heads to a take up reel, which results in dynamic tape stretch with attendant speed fluctuations of the tape across the heads. These perturbations in the tension arise from fluctuations in the supply reel rotational speed, scrape induced tension variations due to rubbing of the tape edges by the supply reel flanges, and frictional effects of heads, guides and rollers located in the tape path.
The propagation of these tension variations through a capstan metered tape drive system has been analyzed in the prior art. In the treatise "Magnetic Recording Handbook", editors C. Denis Mee, Eric D. Daniel, 1990, McGraw-Hill Publishing Company, New York, pp.1107-1112 (J. U. Lemke), it is shown that in a single capstan transport or, in a dual capstan transport wherein the tape velocity is the same at both capstans, tension perturbations present in the output of the tape supply which feeds the capstan assemblies propagate into the head region without attenuation.
In the dual capstan transport having a differential in tape speed between the two capstans, a design value of the tension in the span of tape between the capstans may be set by selection of the speed differential. Generally, this tension must be maintained at a low value to minimize stretch of typically used thin based tape, and under this constraint the supply side tension and its variations also propagate into the tape segment at the heads.
The above results are derived in the referenced analysis, and may be further understood in connection with FIG. 1. A tape supply 10 feeds a tape 12 to a capstan assembly comprising a first capstan 14 and associated pinch roller 16 downstream from the tape supply 10, and then onto a further downstream capstan assembly comprising a second capstan 18 and associated pinch roller 20, after which the tape 12 is spooled onto a takeup reel 22. ("Downstream" means along the tape path in the direction of tape travel, while "upstream" is the direction along the tape path opposite to the direction of tape travel.) Various equivalent means of maintaining contact between the tape and capstan other than by use of a pinch roller are well known in the art. For example, the tape may be driven by frictional forces generated by engaging the tape over a large wrap angle around a capstan having a surface with a high coefficient of friction. Before the tape 12 engages the first capstan 14 it has a velocity v.sub.1,and tension T.sub.1, and upon the tape 12 engaging the second capstan 18 the tape segment 24 has a velocity v.sub.2, and tension T.sub.2. The rotational speed or the diameter of the capstan 18 is configured so that the peripheral velocity at the capstan 18 surface is v.sub.2 =(1+.delta.)v.sub.1. In the analysis referenced above, and ignoring tension loss at the heads, it is shown that the tension T.sub.2 is given by the relation: EQU T.sub.2 =(.delta./.beta.)+(1+.delta.)T.sub.1
where (1+.delta.)=v.sub.2 /v.sub.1 and .beta. is the strain modulus of the tape, equal to .DELTA.l/lT, where l is the tape length and T is the tension.
In a two capstan drive where both capstans impart the same velocity to the tape, or in an equivalent single capstan closed-loop drive, the velocity differential is 0, i.e. in the above relation .delta.=0, and T.sub.2 =T.sub.1. Resultingly, the supply side tape tension and its variations propagate into the tape segment 24 where the heads 26,28 are located.
In the case of the dual capstan differential speed transport, .delta. and .beta. are selected so that EQU (.delta./.beta.)&gt;&gt;(1+.delta.)T.sub.1
and from the above equation, the tension T.sub.2 is effectively set by the dominant term (.delta./.beta.), with the term (1+.delta.)T.sub.1 contributing perturbations in the tension T.sub.2 which arise from variations in T.sub.1. The above results may also be applied to the helical scan drive. The helical scan drive is well known in the video recording art, and it has also been applied in recording large quantities of digital data at high transfer rates. Thin based tape is used to efficiently store the large volume of recorded digital data involved, and this mandates very low tape tensions to obviate tape stretch. Referring to FIG. 2, in a dual capstan helical scan drive, tape 30 feeds from a supply reel 32 through a first capstan 34 and associated pinch roller 36, past guides 38,40 onto the surface of the helical scan drum 42, which it wraps with an angle of .pi. radians. The tape 30 upon leaving the drum 42 passes over guides 44,46 to a second capstan 48 and associated pinch roller 50, and then onto a takeup spool 52. From the previous analysis, it will be appreciated that any tension variations originating on the supply side of the transport, i.e. the supply 32, immediately propagates through the capstan 34 and appears unattenuated as T.sub.in, at the point where the tape 30 engages the drum 42. Typically the drum 42 consists of a lower fixed drum, and a rotating upper drum with the scanner head wheel rotating between the upper and lower drums. As the tape 30 slides across the fixed lower drum of the scan drum 42 the tension rises in the tape 30 due to the friction between the tape and the fixed drum. After leaving the drum 42 the tape tension T.sub.out is determined by the formula T.sub.out= T.sub.in e.sup..mu..theta. (commonly known as the "brake band" equation) where .mu. is the coefficient of friction between the fixed drum and the tape and .theta. is the wrap angle, equal to .pi. radians for the typical helical scan drive. The coefficient of friction .mu. ranges in value from 0.001 to 0.3, and is strongly dependent upon humidity, temperature, and the condition of the drum and tape surfaces. Therefore, the tension T.sub.out, and concurrently as shown in FIG. 3, the total tape stretch resulting from T.sub.out, ##EQU1## can have the widely differing values, e.g. 54 or 56, at a wrap angle of .pi. radians for two different coefficients of friction .mu..sub.1,.mu..sub.2. The resultant tape stretch causes a serious problem in the recording and playback of the digital data. A track may be recorded under conditions of temperature and humidity that result in the coefficient of friction .mu..sub.1, while differing conditions during playback may give rise to the coefficient of friction .mu..sub.2. As previously noted, this causes a differential in tape stretch, and FIG. 4 shows the displacement 60 of a data track during playback due to tape stretch relative to its position 58 during recording. As the head scans the path over the track having displacement 60, it intercepts lesser and lesser amounts of the flux from the track position 58 as recorded. Therefore the envelope of the reproduced signal diminishes during the playback scan, and the available signal to noise deteriorates. This reduced SNR necessitates reduced recording density in order to maintain acceptable accuracy of the reproduced data.